The invention relates to a method of manufacturing plastics film; more specifically, plastics film with fine pores of an uniform diameter by employing controlled high-voltage discharge sparks.
According to the use to which it is put, it is sometimes desirable for plastics film to have a certain degree of air permeability, and it is usual to select the material of which the film is made to have the desired air permeability. However, there are circumstances in which it is impossible to achieve the required degree of air permeability by virtue of the properties of the plastics film material itself. It also sometimes happens that a fixed thickness of film is required for reasons of strength, and this then makes it impossible to obtain the required degree of air permeability. In such cases it is known to create pores in the plastic film by mechanical, electrical, optical or other means. Of these methods, one which has been widely employed in recent years is that in which high-voltage pulses are applied on to the plastics film.
The conventional method of creating pores by means of high-voltage pulses involves running the plastics film between a pair of electrodes, the pores being created by applying high-voltage pulses between the electrodes so as to cause an electrical discharge. In this method, the pore diameter is controlled by the voltage of the high-voltage pulses and the length of time over which they are applied. However, because a plurality of discharge sparks is generated within the duration of one high-voltage pulse in order to create one pore, in the case of comparatively thick film, the discharge spark which creates the pore in the plastics film is generated only after several discharge sparks have been generated, which serve merely to cause the dielectric breakdown of the layer of air between the electrodes until such time as a pore is created in the film. This means that it is impossible to bring about uniform pore diameter simply by controlling the length of time over which the high-voltage pulse is applied. Moreover, the voltage of the high-voltage pulses which are applied between the electrodes is no greater than that which causes the dielectric breakdown of the layer of air between the electrodes and the plastics film. Thus, although the voltage needs to be higher than this, it has hitherto been impossible to control pore diameter by the voltage of the high-voltage pulses.